1. Field of the Invention
The present invention is in the field of tanks, principally for underground fluid storage such as fuel storage, and is particularly directed to double wall tanks comprising an inner tank structure, preferably of steel, for primary fluid containment, an outer tank structure, preferably made of fiber-reinforced resin, for secondary fluid containment, with interstitial space provided between the outer surface of the inner tank structure and the inner surface of the outer tank structure for secondary fluid containment in the event of leakage either outwardly through a breach in the inner tank structure or inwardly through a breach in the outer tank structure.
2. Description of the Prior Art
Regulations of many states and of the U.S. federal government currently require double wall construction for underground fluid storage tanks such as fuel storage tanks, in order to provide secondary fluid containment because of environmental considerations. Such double wall tank construction constitutes, in effect, an outer secondary containment tank shell supported about an inner primary containment tank shell. The inner tank shell defines the primary, inner chamber which provides primary containment for the fluid being stored, while space defined between the inner and outer tank shells, no matter how thin it may be, defines a secondary chamber which provides for secondary containment of the fluid in the event a leak should develop through the wall of the inner tank shell, as for example from corrosion, a faulty weld or resin bond, or from seismic or other mechanical stressing, or through the wall of the outer tank shell. One or more fluid-sensing monitors are conventionally located in communication with one or more low zones in the secondary containment space between the two tanks. Such fluid-sensing monitors are generally located proximate the bottom of the tank proximate end heads of the inner and outer tank structures, in at least one end of the tank, and preferably in both ends. Any leakage outwardly through a breach in the inner primary tank chamber into the secondary containment space, or inwardly through a breach in the secondary containment tank structure into the secondary containment space, is directed toward one or more monitor sensors which then provide an alarm signal to surface equipment indicating the leakage.
There are several different grades and types of underground storage tanks generally considered in the art to be of double wall construction currently in use in the United States, and these are almost all of cylindrical construction and adapted to be layed on their sides underground, i.e., have their cylindrical axes disposed generally horizontally. A full double wall tank consisting of two complete cylindrical tanks, one inside the other, is designated a "double wall" tank. This type of tank has double end walls and 360.degree. double cylindrical wall protection. Most such full double wall tanks are fabricated of steel, although many such full double wall tanks currently produced are non-metallic, as for example having a wound filament fiberglass/resin construction. Improved secondary containment features are applied to such full double wall tanks in applicant's U.S. Pat. No. 4,685,585, issued Aug. 11, 1987, for "Double Wall Tank Manway System," and in applicant's U.S. Pat. No. 4,871,084, issued Oct. 3, 1989, for "Tank Secondary Containment System." While such full double wall tanks provide generally satisfactory secondary containment, they are time consuming and expensive to fabricate, since they require the construction of two complete tanks of different sizes, assembly of the smaller tank inside the larger tank, and welding or resin-bonding of fittings and manways to both the inner and outer tank shells.
Another type of tank which is not completely of double wall construction but is nevertheless commonly referred to as a double wall tank is known as a "wrap tank." In the wrap tank, the primary fluid holding tank is a cylindrical steel tank, with an outer steel sheet provided which gives double wall protection for approximately 330.degree. around the lower part of the tank, leaving the top part of the tank with only the single wall protection of the primary tank. While wrap tanks are cheaper to construct than full double wall tanks, they do not provide secondary containment which is as effective, and regulations in many states such as California do not accept them as double wall tanks.
Another problem in the double wall tank art is that monitor sensing for fluid leakage is conventionally performed at the ends of the tank either between inner and outer end heads of the two tank shells or in external vertical pipes outside the end heads of the tank which extend downwardly from the top of the tank to the bottom and are in communication with the interstitial space between the two tank shells at the bottom of the tank. These are locations which are relatively remote from regions along the length of the tank where a breach would be likely to occur, as for example where pipe fittings and/or manways are welded to the tank structure. It would be desirable for more uniform sensing coverage of potential fluid leaks along the length of the tank to provide sensors at intermediate locations along the length of the tank in the bottom of the cylindrical interstitial space between inner and outer tank shells.
Another problem in the double wall tank art is that cylindrical underground tanks tend to be vulnerable to buckling and to inward collapse of the tank, i.e., implosion, commonly referred to as beam collapse, from external forces on the cylindrical wall of the tank which exceed the beam strength of the tank. This problem of beam collapse failure increases with increased lengths of the tanks, and is of considerable concern for very large underground cylindrical tanks which may be as long as 60 feet.
Another problem in the art is that where at least the inner primary containment tank structure is made of steel, which is usually the case, a breach in the outer secondary containment tank structure, usually of fiber-reinforced resin, will generally admit ground water to the secondary containment zone between the inner and outer shells, and this is likely to initiate corrosion of the inner steel shell. Virtually all of the prior art double wall tanks having monitoring space between the tank walls prior to the present invention have monitoring secondary containment space which is relatively large in volume, usually requiring the accumulation of many gallons of liquid in the secondary containment space before a monitor is energized. Because of this, a relatively small breach in the outer shell can admit water into the secondary containment space for a long period of time, even years, before a monitor sensor is energized, over which time a considerable amount of corrosion of the inner steel tank structure can occur. Similarly, with the usual relatively large volume of the secondary containment space, a breach in the inner tank structure may not be detected for a long period of time after the breach has initiated. Thus, it would be desirable to greatly reduce the volume of the secondary containment space capable of receiving fluid, so as to greatly reduce potential corrosion of a steel inner tank structure and greatly reduce the time required for monitor sensing and signalling after a breach has occured in either the inner primary containment tank structure or the outer secondary containment tank structure.
Regardless of how long a time interval may occur between a breach in the outer tank shell and the signalling of an alarm by a monitor sensor, entry of ground water through a breach in the outer shell can result in rapid initiation and progress of inner steel shell corrosion, particularly where the ground water is substantially acidic, which is a factor that cannot be predicted when a double wall tank is manufactured. In addition to other factors, ground water will frequently tend to be acidic because of dissolved carbon dioxide, making a carbolic acid solution. Therefore, specific protection for a steel inner tank structure against corrosion other than the protection afforded by a potentially breachable resin outer tank shell would be desirable.
Where a large steel structure is continuously immersed in water, such as a ship or drilling platform, zinc bars are conventionally provided proximate the water line as sacrificial anodes to provide cathodic protection for such structures. However, it would not be feasible to employ zinc bars in the narrow confines of the double wall tank secondary containment space, and in any event no practical deployment arrangement can be envisioned which would cover the entire area of the secondary containment space with zinc bars such as to be available for cathodic protection from a breach in the outer shell at any unpredictable location in the shell. Thus, it would be desirable to have cathodic protection for the inner steel tank which covers the entire area of the interstitial secondary containment space.